1. Field of the Invention
The present invention relates to an organic light emitting display device, and more particularly, to an organic light emitting display device having an aperture ratio for each sub-pixel without making a change in driving elements.
2. Description of the Related Art
An organic light emitting display device (OLED), unlike a liquid crystal display (LCD) device, is a self-light emitting display device that does not need a separate light source, and, thus, the OLED can be made thinner. Further, the OLED has advantages in that it is driven with a low voltage to consume less power. Also, the OLED has excellent color expression ability, a high response time, a wide viewing angle, and a high contrast ratio (CR). Therefore, the OLED has been acknowledged as a next-generation display device.
FIG. 1 is a cross-sectional view provided to describe a typical organic light emitting display device. FIG. 1 illustrates a cross section of a sub-pixel SP of a general bottom-emission organic light emitting display device 100 for convenience in explanation. The bottom-emission organic light emitting display device refers to an organic light emitting display device in which a light emitted from an organic light emitting element is released toward the bottom of the organic light emitting display device. Also, the bottom-emission organic light emitting display device refers to an organic light emitting display device in which a light emitted from an organic light emitting element is released toward the bottom of a substrate on which a thin film transistor for driving the organic light emitting display device is formed.
Referring to FIG. 1, a thin film transistor 120 as a driving element for driving an organic light emitting element 130 is formed in a driving area DA on a substrate 110, and an overcoating layer 151 for planarizing an upper part of the thin film transistor 120 and a color filter 140 is formed. An organic light emitting element 130 including an anode 131 electrically connected with thin film transistor 120, an organic light emitting layer 132, and a cathode 133 is formed on the overcoating layer 151. If the organic light emitting display device 100 is a bottom-emission organic light emitting display device, the anode 131 is formed of a transparent conductive material having a high work function value and the cathode 133 is formed of a reflective metallic material having a low work function value. The organic light emitting layer 132 is an organic light emitting layer for emitting white light. Alight emitted from the organic light emitting layer 132 passes through the color filter 140 and is emitted toward the bottom of the substrate 110 on which the thin film transistor 120 is formed. A bank layer 152 is formed on the overcoating layer 151 and defines a light emitting area EA.
Generally, a pixel of an organic light emitting display device includes a plurality of sub-pixels. The respective sub-pixels are configured to emit lights of different colors from each other. Therefore, it is technically important to secure an aperture ratio for each sub-pixel to achieve an optimum luminance for each color. Herein, the aperture ratio refers to a ratio of a light emitting area to a sub-pixel. Optimization of the aperture ratio is closely involved in improving the life of an organic light emitting element.
In the sub-pixel SP of the organic light emitting display device 100 illustrated in FIG. 1, a light emitted from the organic light emitting element 130 cannot be emitted through the driving area DA in which the driving element such as the thin film transistor 120 is formed. Therefore, it is very difficult to secure an aperture ratio in an organic light emitting display device.
A method of reducing the size of a driving area in a sub-pixel may be considered in order to secure an aperture ratio in an organic light emitting display device. However, a driving area in a sub-pixel is optimized by considering the characteristics of an actual product. Therefore, if the number and/or a size of thin film transistors or capacitors disposed in the driving area are reduced to reduce a size of the driving area, reliability of an organic light emitting display device may decrease.
Meanwhile, a method of increasing a size of a pixel may be considered in order to secure an aperture ratio in an organic light emitting display device. However, a size of a pixel in an organic light emitting display device is determined by resolution of the organic light emitting display device. Particularly, in a high-resolution organic light emitting display device, a size of a sub-pixel is very small. Therefore, since the size of a pixel is limited by the resolution of the organic light emitting display device, it is practically impossible to secure an aperture ratio by increasing a size of a pixel.